Hydrogen production by high-temperature water splitting using electron-conducting membranes

ABSTRACT

A device and method for separating water into hydrogen and oxygen is disclosed. A first substantially gas impervious solid electron-conducting membrane for selectively passing hydrogen is provided and spaced from a second substantially gas impervious solid electron-conducting membrane for selectively passing oxygen. When steam is passed between the two membranes at disassociation temperatures the hydrogen from the disassociation of steam selectively and continuously passes through the first membrane and oxygen selectively and continuously passes through the second membrane, thereby continuously driving the disassociation of steam producing hydrogen and oxygen.

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy (DOE)and The University of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates to a method and device for the production ofhydrogen by water splitting.

Global environmental concerns have ignited research to develop energygeneration technologies which leave minimal ecological damage concernsof global climate change are driving nations to develop electric powergeneration technologies and transportation technologies which reducecarbon dioxide emissions. Hydrogen is considered the fuel of choice forboth the electric power and transportation industries.

The need to generate ever larger amounts of hydrogen is clear. Outsideof direct coal liquefaction, other major industrial activities, such aspetroleum refining, also require hydrogen. Collectively, petroleumrefining and the production of ammonia and methanol consumeapproximately 95 percent of all deliberately manufactured hydrogen inthe United States. As crude oil quality deteriorates, and as morestringent restrictions on sulfur, nitrogen and aromatics are imposed,the need for more hydrogen for the refining industry will increase.

Hydrogen production, as a consequence of other processes, issignificant. A number of industries requiring hydrogen produce effluentscontaining significant amounts of unused hydrogen. However, thishydrogen requires clean-up prior to re-use. Furthermore, hydrogen isproduced from the combustion of oil, methane, coal, and otherpetroleum-based materials. However, this hydrogen must be separated fromother combustion gases, namely carbon dioxide, in order to be of use.

Petroleum refineries currently use cryogenics, pressure swing adsorption(PSA), and membrane systems for hydrogen recovery. However, each ofthese technologies have their limitations. For example, because of itshigh costs, cryogenics generally can be used only in large-scalefacilities which can accommodate liquid hydrocarbon recovery.

Membrane-based PSA systems require large pressure differentials acrossmembranes during hydrogen diffusion. This calls for initial compressionof the feed prior to contact to the upstream side of polymeric membranesand recompression of the permeate to facilitate final purificationsteps. Not only are these compression steps expensive, but PSA recoversless feedstream hydrogen and is limited to modest temperatures. U.S.Pat. No. 5,447,559 to Rao discloses a multi-phase (i.e. heterogeneous)membrane system used in conjunction with PSA sweep gases.

Many membrane systems have been developed in efforts to efficientlyextract target material from feed streams. Some of these membranesystems (U.S. Pat. Nos. 5,030,661, 5,645,626, and 5,725,633) aresynthetic based and incorporate polyimides and polyethersulphones. Suchorganic membranes also have limited temperature tolerance.

Proton-exchange membranes have high proton conductivities, and as such,are currently in development for fuel-cell applications and hydrogenpumps. One such application is disclosed in U.S. Pat. No. 5,094,927issued to Baucke on Mar. 10, 1992. However, inasmuch as these membraneshave relatively low electronic conductivities, they are not viable forhydrogen recovery scenarios, primarily because these membranes requirethe application of an electric potential to drive proton transport.

Water disassociates into oxygen and hydrogen at high temperatures, andthe disassociation increases with increasing temperature:$ {H_{2}{O(g)}}\Leftrightarrow{H_{2} + {\frac{1}{2}{O_{2}.}}} $

Because of the small equilibrium constant of this reaction, theconcentrations of generated hydrogen and oxygen are very low even atrelatively high temperatures i.e., 0.1 and 0.042% for hydrogen andoxygen, respectively at 1600° C. However, significant amounts ofhydrogen or oxygen could be generated at moderate temperatures if theequilibrium were shifted toward disassociation. While hydrogen can alsobe produced by high-temperature steam electrolysis, the use of a varietyof membranes including mixed-conducting membranes offers the advantageof requiring no electric power or electrical circuitry. In consideringthe above disassociation equation, it appears at first blush that theremoval of either hydrogen or oxygen would continue to drive thereaction toward disassociation. However, that is not the entire case aswill be hereinafter set forth.

SUMMARY OF THE INVENTION

An object of the invention is to provide a device and method forsplitting water into its component parts wherein the driving force ofthe reaction remains relatively high.

Another object of the invention is to provide a device and method fordisassociating water into oxygen and hydrogen using substantially gasimpervious solid electron-conducting membranes selectively removing thecomponents of the disassociation reaction.

Yet another object of the present invention is to provide a device andmethod for separating water into hydrogen and oxygen in which membranesare used which selectively pass atomic hydrogen or protons on the onehand, and selectively pass atomic oxygen or oxide ions on the otherhand.

Yet another object of the present invention is to provide a device andmethod of water splitting in which either single or two-phase membranesare used selectively to separate hydrogen and oxygen afterdisassociation.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show alternate embodiments of a device for producinghydrogen by high temperature water splitting; and

FIG. 3 is a graphical representation showing the relationship betweenthe partial pressures of oxygen and hydrogen with and without hydrogentransport membranes during steam disassociation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There are a wide variety of membranes which are available thatselectively pass hydrogen either as atomic hydrogen or as protons andare well known in the art. For instance, the Wachsman et al. U.S. Pat.No. 6,235,417 issued May 22, 2001 lists a large number of two-phaseproton and electron conductor membranes of conductive oxides which mayor may not be perovskites. The disclosure of the aforementioned '417patent is herein incorporated in its entirety. In the Wachsman et al.patent, two-phase conductors are shown which are useful in the presentinvention and in which a metal such as palladium is used as anindependent phase in the conductor. However, in addition to palladiumand its alloys, other metals which may be used in this invention includePt, Fe, Co, Cr, Ma, V, Nb, Zr, Ta, V, Ni, Au, Cu, Rh, and Re. Not onlyare the alloys of the aforementioned metallic elements useful as adistinct phase in the membrane for selectively removing hydrogen in thepresent invention, but also various mixtures of the elements and/ortheir alloys and/or their electron-conducting oxides are included in theinvention.

The hydrogen conducting membrane may also include an oxide of the ABO₃formula wherein A is selected from the group consisting of Ba, Ca, Mgand Sr (generally the alkaline earth metals) and B is Ce_(1−x)M_(x) orZr_(1−x)M_(x) or Sn_(1−x)M_(x), where x is greater than zero and lessthan one and M is selected from Ca, Y, Yb, In, Gd, Nd, Eu, Sm, Sr, Mgand Tb. As disclosed in a co-pending patent application Ser. No.09/192,115, filed Nov. 13, 1998 entitled Proton-Conducting MembraneComprising Ceramic, A Method For Separating Hydrogen Using CeramicMembranes, the entire disclosure of which is herein incorporated byreference, mixed oxides of the type disclosed therein in which the oxideis of the general formula ABO₃ wherein A is selected from the groupconsisting of Ba, Ca, Mg and Sr and B is selected from Ce, or Zr, or Sn,which may or may not be doped wherein the dopant is selected from Ca, y,Yb, In, Nd, Gd, Sr and Mg or combinations thereof are also useful in thepresent invention. Moreover, the catalytic metal in the above-disclosedmixed oxides may be selected from Pt, Pd, Fe, Co, Cr, Mn, V, Nb, Zr, Y,Ni, Au, Cu, Rh, Ru, their alloys and mixtures thereof. These membranesare useful for selectively transmitting protons, wherein the membranehas a thickness of between about 0.025 and about 5 millimeters.

In addition to membranes which transmit protons, as illustrated in theaforementioned '417 patent and the aforementioned '115 application,membranes made of certain metals will selectively transport atomichydrogen. These are single phase membranes and include membranes of Pd,Nb, V, Ta, Zr, their alloys and mixtures thereof. Metals such as thoseabove noted may be supported or unsupported. When supported, themembranes may be supported by an oxide or another metal, for instance,alumina as well as yttria stabilized zirconia or SiO₂ may be used asoxide ceramics to support the above-mentioned metals. In addition, othermetals may be used as supports for the above-identified metals, forinstance, Cu may be used as a support metal for Nb.

Where a two-phase hydrogen transmitting membrane is used in which onephase is a mixed oxide ceramic and the other phase is a metal, the metalacts as an electron-conducting portion of the membrane and is generallypreferred to be present in the range of between about 30% by volume andabout 60% by volume. Most preferably, in a two-phase membrane, themembrane is a homogenous mixture of a ceramic oxide and an electronconductor.

To summarize, the hydrogen conducting membrane of the present inventionmay conduct either atomic hydrogen or protons. Where the membraneconducts protons, it may be of the type disclosed in the '417 patent orthe aforementioned '115 application and it is intended that thisinvention will include each and every proton conducting membranedisclosed in either of these documents.

An oxygen conducting membrane is also required in the present invention.The oxygen conducting membranes of the invention may either conductatomic oxygen or oxygen ions, and there are a wide variety of materialswhich function selectively to pass oxygen but not hydrogen. Forinstance, atomic oxygen may be passed through a silver or silver alloymembranes. These membranes like the single phase metal membranesdisclosed for passing hydrogen are single phase materials which may besupported or unsupported and function selectively to pass oxygen in thepresence of hydrogen. Two-phase materials as well as single phasematerials are also function as membranes in the present invention andperovskite oxides as well as other oxides having the general formula ofABO₃ are also suitable as the oxygen passing membrane in the subjectinvention.

European patent application no. 90305654.4 filed May, 24, 1990 by Cableet al, assigned to Standard Oil Company, publication no. A10399833, theentire disclosure of which is herein incorporated by reference,discloses a large number of oxygen ion conducting materials which aresubstantially gas impervious and are multi-phase mixtures ofelectronically conducting material and oxygen ion conductive material.It is intended that the invention disclosed herein cover all themulti-phase membranes disclosed in the aforementioned EPO patentapplication along with single phase materials which transmit oxygenselectively at the operating conditions of the present invention.

More particularly, the present invention includes silver or silveralloys supported and unsupported membranes along with membranes whichare electron-conducting membranes of a mixed metal perovskite oxidehaving a formula ABO₃ wherein A is one or more of the lanthanides, Y andthe rare earth metals and B is one or more of the first row oftransition metals. More particularly, this invention covers oxygenselective membranes which are substantially gas impervious that aretwo-phase materials in which the first phase is a mixed metal oxide ofthe type previously discussed in the sentence above and the second phaseis one or more of Ag, Au, Pt, Rh, Ni, Cu, Ru, Co, their alloys, theirelectron-conducting oxides and mixtures thereof. Most particularly, theselective oxygen conducting membrane of the present invention ispreferably a mixture of Gd doped CeO₂ and Ni and/or a mixture of Y₂O₃stabilized ZrO₂ and Ni. More specifically, the preferred oxygen passingmembrane is GDO_(0.2)Ce_(0.08)O_(2-δ) and Ni is present in the amount ofabout 30 volume percent t about 60 volume percent and most preferablyabout 40 volume percent, and δ is a variable dependent on the extent ofdoping, as is well known in the pertinent art.

Referring now to FIGS. 1 and 2, there are shown devices for practicingthe method of the present invention. More particularly, FIG. 1illustrates a separator 10 in the form of concentric generally tubularmembranes 12 and 14. Membrane 12 is of the type selectively to passatomic hydrogen or protons whereas membrane 14 is of a type selectivelyto pass atomic oxygen or oxygen ions, both as previously described. Eachof the membranes 12 and 14 are as previously described substantially gasimpervious solids, each for selectively passing either hydrogen as inthe case of membrane 12 or oxygen as in the case of membrane 14. Anannulus 16 is formed between the generally tubular membrane 14 and thegenerally tubular membrane 12 and receives steam from a source thereof18 optionally with a pump 20. Preferably, there is a driving forcebetween the gas (steam) in annulus 16 and inside the interior tubularmembrane 14 and the exterior of the larger tubular membrane 12, such asif the steam in the annulus 16 is under pressure. Alternately, theenvironment inside the inner tube 14 may be under vacuum and theenvironment outside the outer tube 12 may be under vacuum or anycombination thereof. Also, the membranes may be reversed with the oxygenpassing membrane interior of the hydrogen passing membrane. Aspreviously indicated, the membranes 12 and 14 may have a thickness inthe range from about 0.002 millimeters to about 5 millimeters, therebeing as is well known in the art, a variety of ways of manufacturingmembranes of the type herein set forth, both supported and unsupported.The invention includes any combination of membranes wherein oneselectively passes hydrogen or protons and the other selectively passesoxygen or oxygen ions to promote the disassociation of steam withoutrequiring external electronic circuitry.

FIG. 2 illustrates schematically as does FIG. 1 a separator 30 having afirst selectively passing hydrogen or proton membrane 32 and a secondmembrane selectively passing oxygen or oxygen ions 34. The membranes 32,34 of the separator 30 may be selected from the same materials aspreviously discussed with respect to the separator 10. There is a space36 formed between the generally flat membrane 32 and the generally flatmembrane 34 into which steam is passed, The steam in the space 36 may beunder pressure as described above with respect to separator 10 or theareas outside of the membranes 32 and 34 may be at reduced pressure orany combination thereof. The separator 30 schematically illustrated inFIG. 2 may be combined with manifolds (not shown) to accommodate aplurality of adjacent parallel membranes 32, 37 to provide a parallelflow device or may be any other art recognized means by which steam athigh temperature can be passed between membranes, each of whichselectively passes either oxygen or hydrogen. The steam disassociationtemperature generally should be at not less than about 700° C. and maybe substantially greater, such as 1500-1600° C., all as dictated byeconomics and engineering decisions.

FIG. 3 shows why the obvious method of simply removing oxygen isinsufficient to drive the disassociation reaction. FIG. 3 shows thepartial pressure of oxygen plotted versus the partial pressure ofhydrogen at 900° C. and at eight tenths atmosphere steam with a hydrogentransporting membrane (HTM) and without a hydrogen transporting membrane(HTM). As seen in FIG. 3, as the hydrogen pressure increases (moving tothe right of the horizontal axis) the partial pressure of oxygendecreases. As the oxygen concentration drops with the increasingconcentration of hydrogen, the driving force of the disassociationreaction of steam diminishes. Therefore, for instance in the separator10, the driving force of the disassociation reaction of steam at theentrance to the separator 10 will be high, but as the steam proceedsaxially along the separator 10, the hydrogen partial pressure increasesdue to disassociation, but the oxygen partial pressure decreases alongwith the driving force for steam disassociation. Therefore, it isinsufficient to remove only oxygen from the disassociation of steam andstill maintain the driving force for steam disassociation. Although notreadily apparent, it is necessary in order to maintain steamdisassociation rates axially of the separator 10 to remove both thehydrogen and the oxygen as they are produced by disassociation of steam.

Although two specific embodiments of separators are disclosed in FIGS. 1and 2 of the present application, it is obvious to those of ordinaryengineering skill in the pertinent art that a variety of designs may beemployed in order to practice the method of the present invention inwhich steam is separated into its constituent parts of hydrogen andoxygen by providing a first substantially gas impervious solidelectron-conducting membrane for selectively passing hydrogen and asecond substantially gas impervious solid electron-conducting membranefor selectively passing oxygen, wherein steam is passed in between thetwo membranes at disassociation temperatures so that as the steamdisassociates, each of its constituent gases is removed selectivelythrough each of the membranes thereby maintaining the driving force ofthe disassociation reaction irrespective of the length of the reactor.

While particular embodiments of the present invention have been shownand described, it will be appreciated by those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects. Therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of the invention, The matter set forth in theforegoing description and accompanying drawings is offered by way ofillustration only and not as a limitation. The actual scope of theinvention is intended to be defined in the following claims when viewedin their proper perspective based on the prior art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A device for separatingwater into hydrogen and oxygen, comprising a first substantially gasimpervious solid electron-conducting membrane for selectively passinghydrogen, a second substantially gas impervious solidelectron-conducting membrane for selectively passing oxygen, andmechanism for passing steam at disassociation temperature between saidfirst and second impervious solid electron-conducting membranes suchthat hydrogen from the disassociation of steam selectively andcontinuously passes through said first substantially gas impervioussolid electron-conducting membrane and oxygen from the disassociation ofsteam selectively and continuously passes through said secondsubstantially gas impervious solid electron-conducting membrane, therebycontinuously driving the disassociation of steam producing hydrogen andoxygen.
 2. The device of claim 1 wherein said first and secondsubstantially gas impervious solid electron-conducting membranes areconcentric tubes forming an annulus with steam in the annulus formed bythe membranes.
 3. The device of claim 2, wherein the steam is maintainedat a positive pressure with respect to the hydrogen and the oxygen. 4.The device of claim 3, wherein the first and second substantially gasimpervious solid electron-conducting membranes each has a thickness inthe range of from about 0.002 and about 5 millimeters.
 5. The device ofclaim 1, wherein said first substantially gas impervious solidelectron-conducting membrane selectively passes atomic hydrogen.
 6. Thedevice of claim 1, wherein said first substantially gas impervious solidelectron-conducting membrane selectively passes protons.
 7. The deviceof claim 1, wherein said first substantially gas impervious solidelectron-conducting membrane is one or more of Pd, Nb, V, Ta, Zr, theiralloys and mixtures.
 8. The device of claim 7, wherein said firstsubstantially gas impervious solid electron-conducting membrane issupported by an oxide ceramic.
 9. The device of claim 8, wherein theoxide ceramic is Al₂O₃ or yttria stabilized zirconia or SiO₂.
 10. Thedevice of claim 7, wherein said first substantially gas impervious solidelectron-conducting membrane is supported by a metal.
 11. The device ofclaim 6, wherein said first substantially gas impervious solidelectron-conducting membrane is an oxide having a formula of ABO₃,wherein A is selected from the group consisting of Ba, Ca, Mg and Sr andSr is Ce_(1−x)M_(x) or Zr_(1−x)M_(x), or Sn_(1−x)M_(x), where X is >0and <1 and M is selected from Ca, Y, Yb, In, Gd, Nd, Eu, Sm, Sr, Mg andTb.
 12. The device of claim 11, wherein an electron conductor is presentas a separate phase in said first substantially gas impervious solidelectron-conducting membrane and is one or more of Pt, Pd, Fe, Co, Cr,Mn, V, Nb, Ta, Zr, Y, Ni, Au, Cu, Rh, Ru, their alloys, theirelectron-conducting oxides, and mixtures thereof.
 13. The device ofclaim 12, wherein the electron conductor is present in said firstsubstantially gas impervious solid electron-conducting membrane in therange of between 30 percent by volume to about 60 percent by volume. 14.The device of claim 12, wherein the first substantially gas impervioussolid electron-conducting membrane is a homogeneous mixture of a ceramicoxide and an electron conductor.
 15. The device of claim 1, wherein saidsecond substantially gas impervious solid electron-conducting membraneis Ag or a Ag alloy.
 16. The device of claim 1, wherein the secondsubstantially gas impervious solid electron-conducting membrane is amixed metal perovskite oxide having a formula of ABO₃ wherein A is oneor more of the lanthanides, Y and the rare earth metals and B is one ormore of the first row of the transition metals.
 17. The device of claim1, wherein said second substantially gas impervious solidelectron-conducting membrane is a two phase material with the firstphase a mixed metal oxide having a formula of ABO₃ and the second phaseone or more of Ag, Au, Pt, Rh, Ni, Cu, Ru, Co, their alloys, theirelectron-conducting oxides and mixtures thereof.
 18. The device of claim1, wherein said second substantially gas impervious solidelectron-conducting membrane is a mixture of Gd doped CeO₂ and Ni. 19.The device of claim 1, wherein said second substantially gas impervioussolid electron conducting membrane is a mixture of Y₂O₃ stabilized ZiO₂and Ni.
 20. A device for separating water into hydrogen and oxygen,comprising a first substantially gas impervious solid membrane solidmembrane of Pd, Nb, V, Ta, Zr, their alloys and mixtures thereof, asecond substantially gas impervious solid membrane of an oxygen-ion andelectron conductor of a mixed metal oxide and/or a perovskite containingone or more lanthanides, Y and the alkaline earth metals, and mechanismfor passing steam at disassociation temperature between said first andsecond substantially gas impervious solid membranes such that hydrogenatoms from the disassociation of steam selectively and continuously passthrough said first substantially gas impervious solid membrane andoxygen ions from the disassociation of steam selectively andcontinuously pass through said second substantially gas impervious solidmembrane, thereby continuously driving the disassociation of steamproducing hydrogen and oxygen.
 21. The device of claim 20, wherein saidfirst substantially gas impervious solid membrane is supported by anoxide ceramic.
 22. The device of claim 20, wherein said firstsubstantially gas impervious solid membrane is supported by a metal. 23.The device of claim 20, wherein said second substantially gas impervioussolid oxygen-ion and electron-conductor membrane is a homogeneousmixture of ceramic oxide and an electron conductor.
 24. The device ofclaim 23, wherein said second substantially gas impervious solidoxygen-ion and electron-conductor membrane is a two phase material withthe first phase a mixed metal oxide perovskite having a formula of ABO₃and the second phase one or more of Ag, Au, Pt, Rh, Ni, Cu, Ru, Co,their alloys, their electron-conducting oxides and mixtures thereof. 25.The device of claim 20, wherein said second substantially gas impervioussolid oxygen-ion and electron-conductor membrane is mixture of Gd dopedceria and Ni.
 26. The device of claim 25, wherein Ni is present in saidsecond substantially gas impervious solid oxygen-ion andelectron-conducting membrane in the range of from about 30 to about 60percent by volume.
 27. The device of claim 20, wherein said secondsubstantially gas impervious solid oxygen-ion and electron-conductormembrane is a mixture of Gd_(0.2)Ce_(0.8)O_(2-δ) and Ni, wherein the Niis present in amount of about 40 percent by volume, and δ is variable.28. The device of claim 20, wherein said first substantially gasimpervious solid electron-conducting membrane includes Pd, and alloysthereof.
 29. The device of claim 20, wherein said first and secondsubstantially gas impervious solid electron-conducting membranes areconcentric tubes.
 30. A method of separating water into hydrogen andoxygen, comprising providing a first substantially gas impervious solidelectron-conducting membrane for selectively passing hydrogen, providinga second substantially gas impervious solid electron-conducting membranefor selectively passing oxygen, and passing steam at disassociationtemperature between the first and second impervious solidelectron-conducting membranes such that hydrogen from the disassociationof steam selectively and continuously passes through the firstsubstantially gas impervious solid electron-conducting membrane andoxygen from the disassociation of steam selectively and continuouslypasses through the second substantially gas impervious solidelectron-conducting membrane, thereby continuously driving thedisassociation of steam producing hydrogen and oxygen.
 31. The method of30, wherein the steam is maintained at a temperature not less than about700° C.
 32. The method of claim 31, wherein said first substantially gasimpervious solid electron-conducting membrane is one or more of Pd, Nb,V, Ta, Zr, their alloys and mixtures thereof.
 33. The method of claim32, wherein atomic hydrogen is passed by the first membrane.
 34. Themethod of claim 32, wherein protons are passed through the firstmembrane and oxygen ions are passed through the second membrane.
 35. Themethod of claim 30, wherein the second substantially gas impervioussolid electron-conducting membrane is a mixed metal perovskite oxidehaving a formula of ABO₃ wherein A is one or more of the lanthanides, Yand the rare earth metals and B is one or more of the first row of thetransition metals.
 36. The method of claim 30, wherein the secondsubstantially gas impervious solid electron-conducting membrane is amixture of Gd doped CeO₂ and Ni or a mixture of Y₂O₃ stabilized ZrO₂ andNi.